At 1800 RPM the engine is pretty darn quiet. I mean no louder than the old Onan. Plus I already carry 110 gallons of diesel. I am half way to my idea, just need to buy a belt driven generator head. The big difference also being how much lighter this set up is, basically half the weight of the Onan.
It sounds like you're planning to run an ambulance 275 amp alternator, then to batteries or to an inverter. That way, the exact speed isn't crucial since you're not trying to hold 60 Hz. Good plan!.
A slow diesel will be a lot quieter than a 3600 RPM Yanmar. The only GOTTCHA you might think about is planning the suction tap from the fuel tanks so sitting on the diesel can't run you out of fuel for the truck.